rewrite unitsquare(n,'square') using rectilinear
rewrite unitsquare(n,'square') using rectilinear

The old implementation of `mesh.unitsquare` with square elements directly
instantiates a `StructuredTopology`. In future patches tensorial topologies
will be introduced and `mesh.rectilinear` will generate a tensorial topology if
some environment variable is set. In order to prevent code duplication, this
patch uses `rectilinear` instead of `StructuredTopology`. When the time has
come, `unitsquare` with square elements will automatically support tensorial
topologies.

wijnandhoitinga/nutils